# Forum

### Author Topic: Placing markers  (Read 8362 times)

#### PROBERT1968

• Sr. Member
• Posts: 370
• Hello !
##### Placing markers
« on: May 22, 2019, 08:03:51 PM »
I do have a question for you.  I am reviewing reading the manual of Metashape 1.5  and notice that under "Placing markers" on page 59.

The quote comes from the book that says " The more photos are used to specify marker position the higher is the accuracy of marker placement. To define the marker location within a scene, it should be placed on at least 2 photos

Kind of confuse here but my two questions here...

1) Does it mean place the markers at least 2  each photos ?

2) OR Does it mean to  placed at least 2 photos all together all set one of photos ?

For example, if you have say 6 photos that you need to do orthomosic, etc,  Does that mean I have to do at least 2 markers for 6 photos... So that is about at least 12 markers ?

or for 6 photos you have about 6 markers in total whole photos ?

Thank you

#### Kiesel

• Sr. Member
• Posts: 332
##### Re: Placing markers
« Reply #1 on: May 24, 2019, 09:58:48 AM »
Hi PROBERT1968,

Quote
" The more photos are used to specify marker position the higher is the accuracy of marker placement. To define the marker location within a scene, it should be placed on at least 2 photos "

means that it is not enough to place a marker in only one photo to calculate the postition in 3D because a photo is only 2D. So it is necessary to place the same marker in at least two photos to calculate its 3D position and its position in all other photos by Metashape. After that you should check the position of that marker if it is correct placed in all photos.

Hopes this makes it a little bit clearer to you.

Kiesel
« Last Edit: May 29, 2019, 10:01:52 AM by Kiesel »

#### PROBERT1968

• Sr. Member
• Posts: 370
• Hello !
##### Re: Placing markers
« Reply #2 on: May 25, 2019, 05:59:51 PM »
Thank you Kiesel

so meaning that one photo should be placed at least 2 markers  if I understand right ?

#### James

• Hero Member
• Posts: 763
##### Re: Placing markers
« Reply #3 on: May 28, 2019, 12:25:27 AM »
No, it is only referring to individual markers.

A marker placed in only one photo simply defines a 'ray' i.e. a line originating at the camera location and extending to infinity on the direction of the position it was placed on the photo.

That same marker placed in a second photo defines another 'ray' and where those rays intersect, is the 'position' of the marker in 3d space.

These rays are also referred to as projections in metashape/photoscan and the same applies to image tie points as well.

Without having two rays/projections defined by the same marker in at least two images, there is no way to infer the 3d position, and so any marker with less than two projections (occurrence in two images) is basically useless.

Furthermore, it is most likely that two rays will actually not intersect at all, due to imprecision in placement and inaccuracy of estimated camera orientations, so the 3d point is actually the point where the rays come closest to intersecting. If you then go on to place that same marker in a third, forth, fifth image and so on, the number of projections increases respectively and so the 3d point is where these multiple projections come closest to all intersecting. If the markers are placed precisely, then this greater number of projections/rays should help to increase the accuracy of the marker.

If the cameras were not well aligned then after adding a marker to multiple images in this way hitting the optimise button should adjust alignment to bring the projections to a closer intersection.

All this only applies to a single marker, placed in multiple photos. I.e. "point 1" in photos "DSC_0001.JPG", "DSC_0002.JPG" and "DSC_0003.JPG" means that "point 1" has 3 projections.

It will generally be necessary to use multiple markers in a project but how many you need, and how many you need in each photo will depend on what you are trying to achieve and what kind of photo set you are working with.

#### Kiesel

• Sr. Member
• Posts: 332
##### Re: Placing markers
« Reply #4 on: May 29, 2019, 10:17:37 AM »
Hi PROBERT1968,

No, it means that the SAME marker should be placed in at least two, better more, photos.

But James has already explained it very well in more detail.

Kiesel
« Last Edit: May 29, 2019, 11:07:25 AM by Kiesel »

#### PROBERT1968

• Sr. Member
• Posts: 370
• Hello !
##### Re: Placing markers
« Reply #5 on: May 29, 2019, 04:35:49 PM »
Thank you everyone !  It would be nice for each of you to get a credit !